An IC package such as a CPU which is an electric component mounted on a printed circuit board, is liable to suffer decreased performance or damage due to the temperature rise occasioned by the heat emitted during use. In the prior art, a heat dissipating sheet or heat dissipating grease with good thermoconducting properties was used between the IC package and a heat dissipating fin. However, the increased performance of electronic components has led to more and more heat emission, so a substance with more powerful thermoconducting properties was desired.
Although the dissipating sheets of the prior art are easily mounted, as far as regards the ease of processing steps in their manufacture, there is a limit to the filler amount they can contain. Consequently, they had the disadvantage that sufficient thermoconducting properties could not be obtained. Heat dissipating grease on the other hand can follow and be in intimate contact with the surfaces of the CPU or heat dissipating fins without being affected by imperfections on the surface, but it soils other components, and oil flows out from it after long periods of use.
A method has been proposed wherein a liquid silicone rubber composition is used as a potting agent or adhesive, and another method has been proposed for curing and interposing a heat dissipating grease (Japanese Patent Application Public Disclosure Sho 61-157569, Japanese Patent Application Public Disclosure Hei 8-208993). However, as emphasis was given to ease of operation when the heat dissipating grease was applied, there was a limit to the blending amount of thermoconducting filler which was blended in order to impart thermoconducting properties. As a means of improving thermoconducting properties, a method has been proposed for melting and dispersing a low melting point metal or alloy which is a solid at ordinary temperature, in the silicone composition (Japanese Patent Application Public Disclosure Hei-7-207160, U.S. Pat. No. 5,445,308). However, as this crushed low melting point metal or alloy is melted in order to blend it, it lacks uniform dispersion properties so the thermoconducting silicone composition is non-homogeneous, and as the thermoconducting silicone composition contains powder of large particle diameter due to the low melting point metal, a thin film could not be applied.
The inventors carried out intensive studies in order to resolve the above problems. As a result, it was discovered that using a thermoconducting silicone composition as a filler, this composition being a combination of a low melting point metal powder which is a solid at ordinary temperature having a controlled particle size and a thermoconducting filler, imparted excellent heat dissipating properties. It was found that when the low melting point metal was blended and kneaded at a temperature below the melting point, the thermoconducting silicone composition was very homogeneous and had a high thermoconductivity. It was also found that the thermal resistance of the thermoconducting silicone composition could be very much lowered by transiently applying heat to raise the temperature above the melting point of the low melting point metal powder after curing.
It is therefore a first object of this invention to provide a thermoconducting silicone composition having excellent heat dissipating properties when used as a heat dissipating structure of a semiconductor device.
It is a second object of this invention to provide a thermoconducting silicone composition with thermoconducting properties improved by post-heating.
It is a third object of this invention to provide a method of introducing the aforesaid thermoconducting silicone composition between an IC package and a heat dissipating fin.It is a fourth object of this invention to provide a heat dissipating structure for a semiconductor device having excellent heat dissipating properties.